Amt airframe hb_vol_1-32-74 paginas 32 a 74

Figure 1-11. Examples of different categories of aircraft, clockwise from top left: lighter-than-air, glider, rotorcraft, and airplane.
Figure 1-12. Amonoplane (top), biplane (middle), and tri-wing
aircraft (bottom).
coverage of the unique airframe structures and
maintenance practices for lighter-than-air flying machines
is not included in this handbook.
The airframe of a fixed-wing aircraft consists offive principal
units:the fuselage,wings,stabilizers, flight control surfaces,
and landing gear. [Figure1-13] Helicopter airframes consist
of the fuselage,main rotor and related gearbox, tail rotor (on
helicopters with a single main rotor), and the landing gear.
Airframe structuralcomponents are constructed froma wide
variety of materials. The earliest aircraft were constructed
primarily of wood. Steel tubing and the most common
material, aluminum, followed. Many newly certified aircraft
are built from molded composite materials, such as carbon
fiber. Structural members of an aircraft’s fuselage include
stringers, longerons, ribs, bulkheads, and more. The main
structural member in a wing is called the wing spar.
The skin of aircraft can also be made from a variety of
materials, ranging from impregnated fabric to plywood,
aluminum, orcomposites.Underthe skin and attached to the
structural fuselage are the many components that support
airframe function.The entire airframe and its components are
joined by rivets,bolts,screws,and other fasteners. Welding,
adhesives, and special bonding techniques are also used.
Major Structural Stresses
Aircraft structuralmembers are designed to carry a load or to
resist stress. In designing an aircraft, every square inch of
wing and fuselage,every rib,spar,and eveneach metalfitting
must be consideredin relation to the physical characteristics
of the material of which it is made. Every part of the aircraft
must be planned to carry the load to be imposed upon it.
1-6

Wings
Flight
controls
Pow erplant
Stabilizers
Fuselage
Flight controls
Landing gear
Figure 1-13. Principal airframe units.
The determi-nation of such loads is called stress analysis.
Although planning the design is not the function of the aircraft
technician, it is, nevertheless, important that the technician
understand and appreciate the stresses involved- in order to avoid
changes in the original design through improper repairs.
The term “stress” is often used interchangeably with the
word “strain.” While related, they are not the same thing.
External loads or forces cause stress. Stress is a material’s
internal resistance, or counterforce, that opposes
deformation. The degree of deformation of a material is
strain. When a material is subjected to a load or force, that
material is deformed, regardless of how strong the material
is or how light the load is.
There are five major stresses [Figure 1-14] to which all
aircraft are subjected:
• Tension
• Compression
• Torsion
• Shear
• Bending
Tension is the stress that resists a force that tends to pull
something apart. [Figure 1-14A] The engine pulls the aircraft
forward, but air resistance tries to hold it back. The result is
tension, which stretches the aircraft. The tensile strength of a
material is measured in pounds per square inch (psi) and is
calculated by dividing the load (in pounds) required- to pull the
material apart by its cross-sectional- area (in square inches).
Compression is the stress that resists- a crushing force.
[Figure 1-14B] The compressive strength of a material is
also measured in psi. Compression is the stress that tends
to shorten or squeeze aircraft parts.
Torsion is the stress that produces twisting. [Figure 1-
14C] While moving the aircraft forward, the engine- also
tends to twist it to one side, but other aircraft components
hold it on course. Thus, torsion is created. The torsion
strength of a material is its resistance to twisting or torque.
Shear is the stress that resists the force tending to cause
one layer of a material to slide over an adjacent layer.
[Figure 1-14D] Two riveted plates in tension subject the
rivets to a shearing force. Usually, the shearing strength of
a material is either equal to or less than its tensile or
compressive strength. Aircraft parts, especially screws,
bolts, and rivets, are often subject to a shearing force.
Bending stress is a combination ofcompression and tension.
The rod in Figure 1-14E has been shortened-(compressed)on
1-7

A. Tension
B. Compression
C. Torsion D. Shear
Tension outside of bend
Bent structuralmember Shear along imaginary line (dotted)
Compression inside of bend
E. Bending (the combination stress)
Figure 1-14. Thefive stresses that may act on an aircraft and its parts.
the inside of the bend and stretched on the outside of the
bend. A single member of the structure may be subjected
to a combination of stresses. In most cases, the structural-
members are designed to carry end loads rather than side
loads. They are designed to be subjected to tension or
compression rather than bending.
Strength or resistance to the external loads imposed during
operation may be the principal requirement in cer-tain
structures.However,there are numerousothercharacteristics
in addition to designingto controlthe five major stresses that
engineers must consider. For example, cowling, fairings, and
similar- parts may not be subject to significant loads requiring
a high degree of strength. However, these parts must have
streamlined shapes to meet aerodynamic requirements, such
as reducing drag or directing airflow.
1-8
Fixed-Wing Aircraft
Fuselage
The fuselage is the main structure or body of the fixed-wing
aircraft. It provides space for cargo, controls, accessories,-
passengers,and otherequipment.In single-engine aircraft,the
fuselage houses the powerplant. In multiengine aircraft, the
engines may be either in the fuselage, attached to the
fuselage,orsuspendedfromthe wing structure.There are two
generaltypes offuselageconstruction:-truss and monocoque.
Truss-Type
A truss is a rigid framework made up of members, such as
beams, struts, and bars to resist deformation- by applied loads.
The truss-framed fuselage is generally covered with fabric. The
truss-type fuselage frame is usually constructed of steel

tubing welded together in such a manner that all members of
the truss can carry both tension and compression loads.
[Figure 1-15] In some aircraft, principally the light, single-
engine models,truss fuselage frames may be constructed of
aluminum alloy and may be riveted or bolted into one piece,
with cross-bracing achieved by using solid rods or tubes.
Monocoque Type
The monocoque (single shell) fuselage relies largely on the
strength of the skin or covering to carry the primary loads.
The design may be divided- into two classes:
1. Monocoque
2. Semimonocoque
Different portions- of the same fuselage may belong to
either of the two classes, but most modern aircraft are
considered to be of semimonocoque type construction.
The true monocoque construction uses formers, frame
assemblies, and bulkheads to give shape to the fuselage.
[Figure 1-16] The heaviest of these structural members are
located at intervals to carry concentrated loads and at points
where fittings are used to attach other units such as wings,
powerplants,and stabilizers.Since no other bracing members
are present,the skin must carry the primary stresses and keep
the fuselage rigid. Thus, the biggest problem involved in
monocoque- construction is maintaining enough strength
while keeping the weight within allowable limits.
Semimonocoque Type
To overcome the strength/weight problem of monocoque
construction, a modification called semi-monocoque
construction was developed-. It also consists of frame
assemblies,bulkheads,andformers as usedin the monocoque
design but,additionally,the skin is reinforced by longitudinal
Longeron
Diagonal w eb members
Verticalw eb members
Figure 1-15. A truss-type fuselage. A Warren truss uses mostly
diagonal bracing.
Skin Former
Bulkhead
Figure 1-16. An airframe using monocoque construction.
members called longerons.Longerons usually extend across
several frame members and help the skin support primary
bending loads. They are typically made of aluminum alloy
either of a single piece or a built-up construction.
Stringers are also used in the semimonocoque fuselage. These
longitudinal members are typically more numerous and lighter in
weight than the longerons. They come in a variety of shapes and
are usually made from single piece aluminum alloy extrusions or
formed aluminum. Stringers have some rigidity but are chiefly
used for giving shape and for attachment of the skin. Stringers
and longerons together prevent tension and compression from
bending the fuselage. [Figure 1-17]
Longeron Skin
Stringer
Bulkhead
Figure 1-17. The most common airframe construction is
semimonocoque.
1-9

Otherbracing between the longeronsand stringers canalso be
used. Often referred to as web members, these additional
support pieces may be installed vertically or diagonally. It
must be noted thatmanufacturersuse different nomenclature
to describe structural members. For example, there is often
little difference between some rings, frames, and formers.
One manufacturermay call the same type of brace a ring or a
frame. Manufacturer instructions and specifications for a
specific aircraft are the best guides.
The semimonocoque fuselage is constructed primarily of alloys
of aluminum and magnesium, although steel and titanium are
sometimes found in areas of high temperatures. Individually, not
one of the aforementioned components is strong enough to carry
the loads imposed during flight and landing. But, when
combined, those components form a strong, rigid framework.
This is accomplished with gussets, rivets, nuts and bolts, screws,
and even friction stir welding. A gusset is a type of connection
bracket that adds strength. [Figure 1-18]
To summarize, in semimonocoque fuselages, the strong,
heavy longerons hold the bulkheads and formers, and these,
in turn,hold the stringers, braces, web members, etc. All are
designedto be attached togetherand to the skin to achieve the
full-strength benefits of semimonocoque design. It is
important to recognize that the metal skin or covering carries
part of the load.The fuselage skin thicknesscan vary with the
load carried and the stressessustained at a particularlocation.
The advantages of the semimonocoque fuselage are many.
The bulkheads,frames,stringers,and longerons facilitate the
design-and construction ofa streamlined fuselage that is both
rigid and strong.Spreadingloadsamong these structures and
the skin means no single piece is failure critical. This means
that a semimonocoque fuselage, because of its stressed-skin
construction,may withstand-considerable damageand stillbe
strong enough to hold together.
Fuselages are generally constructed in two or more
sections. On small aircraft, they are generally made in two
or three sections, while larger aircraft may be made up of
as many as six sections or more before being assembled.
Pressurization
Many aircraft are pressurized. This means that air is pumped
into the cabin after takeoff and a difference in pressure
between the airinside the cabin and the air outside the cabin
is established.This differentialis regulatedand maintained.In
this manner,enoughoxygen is made available forpassengers
to breathe normally and move around the cabin without
special equipment at high altitudes.
Pressurization causes significant stress on the fuselage
structure and adds to the complexity of design. In addition
to withstanding the difference in pressure between the air
inside and outside the cabin, cycling fromunpressurized to
pressurized and back again each flight causes metal
fatigue. To deal with these impacts and the other stresses
of flight, nearly all pressurized aircraft are
semimonocoque in design. Pressurized fuselage structures
undergo extensive periodic inspections to ensure that any
damage is discovered and repaired. Repeated weakness or
failure in an area of structure may require that section of
the fuselage be modified or redesigned.
Wings
Wing Configurations
Wings are airfoils that, when moved rapidly through the air,
create lift. They are built in many shapes and sizes. Wing
design can vary to provide certain desirable flight
characteristics. Control at various operating speeds, the
amount oflift generated, balance, and stability all change as
the shape ofthe wing is altered.Both the leading edge andthe
trailing edge of the wing may be straight or curved, or one
edge may be straight and the othercurved.One orboth edges
may be tapered so that the wing is narrower at the tip than at
the root where it joins the fuselage. The wing tip may be
square, rounded, or even pointed. Figure 1-19 shows a
number of typical wing leading and trailing edge shapes.
The wings of an aircraft can be attached to the fuselage at
the top, mid-fuselage, or at the bottom. They may extend
perpendicular to the horizontal plane of the fuselage or can
angle up or down slightly. This angle is known as the wing
dihedral. The dihedral angle affects the lateral stability of
the aircraft. Figure 1-20 shows some common wing attach
points and dihedral angle.
Figure 1-18. Gussets are used to increase strength.
1-10

Tapered leading edge, Tapered leading and Delta w ing
straight trailing edge trailing edges
Sw eptbackwings
Straight leading and Straight leading edge,
trailing edges tapered trailing edge
Figure 1-19. Various wing design shapes yield different performance.
Low wing Dihedral
High w ing Mid w ing
Gull w ing Inverted gull
Figure 1-20. Wing attach points and wing dihedrals.
Wing Structure
The wings of an aircraft are designed to lift it into the air.
Their particular design for any given aircraft depends on a
number of factors, such as size, weight, use of the aircraft,
desired speed in flight and at landing, and desired rate of
climb. The wings of aircraft are designated left and right,
corresponding to the left and right sides of the operator
when seated in the cockpit. [Figure 1-21]
Often wings are of full cantilever design. This means they
are built so that no external bracing is needed. They are
supported internally by structural members assisted by the
skin of the aircraft. Other aircraft wings use external struts
or wires to assist in supporting the wing and carrying the
aerodynamic and landing loads. Wing support cables and
struts are generally made from steel. Many struts and their
1-11

Left w ing
Right w ing
Figure 1-21. “Left” and “right” on an aircraft are oriented to the perspective of a pilot sitting in the cockpit.
attach fittings have fairings to reduce drag. Short, nearly
vertical supports called jury struts are found on struts that
attach to the wings a great distance fromthe fuselage. This
serves to subdue strut movement and oscillation caused by
the air flowing around the strut in flight. Figure 1-22
shows samples of wings using external bracing, also
known as semicantilever wings. Cantilever wings built
with no external bracing are also shown.
Aluminum is the most common material from which to
construct wings, but they can be wood covered with fabric,
and occasionally a magnesium alloy has been used.
Moreover, modern aircraft are tending toward lighter and
stronger materials throughout the airframe and in wing
construction. Wings made entirely of carbon fiber or other
composite materials exist, as well as wings made of a
combination of materials for maximum strength to weight
performance.
The internal structures of most wings are made up of spars
and stringers running spanwise and ribs and formers or
bulkheads running chordwise (leading edge to trailing
edge). The spars are the principle structural members of a
wing. They support all distributed loads, as well as
concentrated weights such as the fuselage, landing gear,
and engines. The skin, which is attached to the wing
structure, carries part of the loads imposed during flight. It
also transfers the stresses to the wing ribs. The ribs, in
turn, transfer the loads to the wing spars. [Figure 1-23]
In general, wing construction is based on one of three
fundamental designs:
1. Monospar
2. Multispar
3. Box beam
Modification of these basic designs may be adopted by
various manufacturers.
The monosparwing incorporates only onemain spanwise or
longitudinalmemberin its construction.Ribs orbulkheads
Full cantilever
Wire braced biplane
Semicantilever
Long struts braced with jury struts
Figure 1-22. Externally braced wings, also called semicantilever wings, havewires or struts to support the wing. Full cantilever wings
have no external bracing and are supported internally.
1-12

Ribs Rear spar
Nose rib
Front spar
Figure 1-23. Wing structure nomenclature.
Stringer
Ribs
Skin
supply the necessary contour or shape to the airfoil.
Although the strict monospar wing is not common, this
type of design modified by the addition of false spars or
light shear webs along the trailing edge for support of
control surfaces is sometimes used.
The multispar wing incorporates more than one main
longitudinal member in its construction. To give the wing
contour, ribs or bulkheads are often included.
The box beam type of wing construction uses two main
longitudinal members with connecting bulkheads to furnish
additionalstrength andto give contourto the wing. [Figure 1-
24] A corrugatedsheet may be placed between the bulkheads
and the smooth outer skin so that the wing can better carry
tension and compression loads. In some cases, heavy
longitudinalstiffeners are substitutedforthe upper surface of
the wing and stiffeners on the lower surface
corrugated sheets. A combination of corrugated sheets on
the upper surface of the wing and stiffeners on the lower
surface is sometimes used. Air transport category aircraft
often utilize box beam wing construction.
Wing Spars
Spars are the principal structural members of the wing. They
correspond to the longerons of the fuselage-. They run
parallel to the lateral axis of the aircraft, from the fuselage
toward the tip of the wing, and are usually attached to the
fuselage by wing fittings, plain beams, or a truss.
Spars may be made of metal, wood, or composite materials
depending on the design criteria of a specific aircraft.
Wooden spars are usually made from spruce. They can be
generally classified into four different types by their cross-
sectionalconfiguration-.As shown in Figure 1-25, they may
be (A) solid, (B) box-shaped, (C) partly hollow, or (D) in
Figure 1-24. Box beam construction.
1-13

A B C D E
Figure 1-25. Typical wooden wing spar cross-sections.
the form of an I-beam. Lamination of solid wood spars is
often used to increasestrength. Laminated wood can also be
found in box-shapedspars.The sparin Figure 1-25E has had
material removed to reduce weight but retainsthe strength of
a rectangular spar. As can be seen, most wing spars are
basically rectangularin shapewith the long dimension of the
cross-section oriented up and down in the wing.
Currently, most manufactured aircraft have wing spars
made of solid extruded aluminum or aluminum extrusions
riveted together to form the spar. The increased use of
composites and the combining of materials should make
airmen vigilant for wings spars made from a variety of
materials. Figure 1-26 shows examples of metal wing spar
cross-sections.
In an I–beam spar, the top and bottom of the I–beam are
called the caps and the verticalsection is called the web. The
entire sparcan be extruded fromone piece of metal but often
it is built up frommultiple extrusions or formed angles. The
web forms the principaldepth portion ofthe spar and the cap
strips (extrusions, formed angles, or milled sections) are
attached to it. Together, these members carry
the loads caused by wing bending, with the caps providing
a foundation for attaching the skin. Although the spar
shapes in Figure 1-26 are typical,- actual wing spar
configurations- assume many forms. For example, the web
of a sparmay be a plate or a truss as shown in Figure 1-27.
It could be built up from lightweight materials with
vertical stiffeners employed for strength. [Figure 1-28]
It could also have no stiffeners but might contain flanged
holes for reducing weight but maintaining strength. Some
metal and composite wing spars retain the I-beam concept
but use a sine wave web. [Figure 1-29]
Additionally,fail-safe sparweb design exists.Fail-safe means
that should one member of a complex structure fail, some
other part of the structure assumes the load of the failed
member and permits continued operation. A spar with fail-
safe constructionis shownin Figure 1-30. This spar is made
in two sections. The top section consists of a cap riveted to
the upper web plate. The lower section is a single extrusion
consisting ofthe lowercap and web plate.These two sections
are spliced together to formthe spar. If either section of this
Figure 1-26. Examples of metal wing spar shapes.
1-14

Upper cap member
Diagonal tube
Verticaltube
Low er cap member
Figure 1-27. Atruss wing spar.
Upper spar cap
Stiffener
Rib attach angle
Low er spar cap
Figure 1-28. Aplate web wing spar with vertical stiffeners.
Sine w ave web
Caps
Figure 1-29. A sine wave wing spar can be made from aluminum
or composite materials.
type ofsparbreaks,the othersectioncan stillcarry the load.
This is the fail-safe feature.
As a rule, a wing has two spars. One spar is usually located
nearthe front of the wing, and the other about two-thirds of
the distance toward the wing’s trailing edge. Regardless of
type, the spar is the most important part of the wing. When
Upper spar cap
Rivets
Splice
Upper
spar
Lower
web
spar
web
Low er spar cap
Figure 1-30. Afail-safe spar with a riveted spar web.
other structural members of the wing are placed under load,
most of the resulting stress is passed on to the wing spar.
False spars are commonly used in wing design. They are
longitudinalmembers like spars but do not extend the entire
spanwise length of the wing. Often, they are used as hinge
attach points for control surfaces, such as an aileron spar.
Wing Ribs
Ribs are the structural crosspieces that combine with spars
and stringers to make up the framework of the wing. They
usually extend from the wing leading edge to the rear spar
or to the trailing edge of the wing. The ribs give the wing
its cambered shape and transmit the load fromthe skin and
stringers to the spars. Similar ribs are also used in ailerons,
elevators, rudders, and stabilizers.
Wing ribs are usually manufactured from either wood or
metal. Aircraft with wood wing spars may have wood or
metal ribs while most aircraft with metal spars have metal
ribs. Wood ribs are usually manufactured fromspruce. The
three most common types of wooden ribs are the plywood
web,the lightened plywood web,and the trusstypes.Ofthese
three,the truss type is the most efficient because it is strong
and lightweight,but it is also the most complexto construct.
Figure 1-31 shows wood truss web ribs and a lightened plywood
web rib. Wood ribs have a rib cap or cap strip fastened around
the entire perimeter of the rib. It is usually made of the same
material as the rib itself. The rib cap stiffens and strengthens the
rib and provides an attaching surface for the wing covering. In
Figure 1-31A, the cross-section of a wing rib with a truss-type
web is illustrated. The dark rectangular sections are the front and
rear wing spars. Note that to reinforce the truss, gussets are used.
In Figure 1-31B, a truss web rib is shown with a continuous
gusset. It provides greater support throughout the entire rib with
very little additional
1-15

A
B
C
Figure 1-31. Examples of wing ribs constructed of wood.
weight.A continuousgusset stiffensthe cap strip in the plane
of the rib. This aids in preventingbuckling andhelpsto obtain
betterrib/skin joints where nail-gluing is used.Such a rib can
resist the driving force of nails better than the other types.
Continuous gussets are also more easily handled than the
many small separate gussets otherwise required. Figure 1-
31C showsa rib with a lighten plywood web.It also contains
gussetsto support the web/capstrip interface.The cap strip is
usually laminated to the web, especially at the leading edge.
A wing rib may also be referred to as a plain rib or a main rib.
Wing ribs with specialized locations or functions are given
names that reflect theiruniqueness.For example, ribs that are
located entirely forward of the front spar that are used to
shape and strengthen the wing leading edge are called nose
ribs or false ribs.False ribs are ribs that do not spanthe entire
wing chord,which is the distance fromthe leading edge to the
trailing edge ofthe wing.Wing butt ribs may be found at the
inboard edge of the wing where the wing attaches to the
fuselage. Depending on its location and method of
attachment, a butt rib may also be called a bulkhead rib or a
compressionrib if it is designedto receive compressionloads
that tend to force the wing spars together.
Since the ribs are laterally weak, they are strengthened in
some wings by tapes that are woven above and below rib
sections to prevent sidewise bending of the ribs. Drag and
anti-drag wires may also be found in a wing. In Figure 1-
32, they are shown criss-crossed between the spars to form
a truss to resist forces acting on the wing in the direction
of the wing chord. These tension wires are also referred to
as tie rods. The wire designed to resist the backward-
forces is called a drag wire; the anti-drag wire resists the
forward forces in the chord direction. Figure 1-32
illustrates the structural components of a basic wood wing.
At the inboard end of the wing spars is some formof wing
attach fitting as illustrated in Figure 1-32. These provide a
strong and secure method for attaching the wing to the
fuselage.The interface betweenthe wing and fuselageis often
Leading edge strip Nose rib or false rib
Wing tip Front spar Anti-drag w ire or tie rod Drag w ire or tie rod Wing attach fittings
False spar or aileron spar Rear spar Wing rib or plain rib
Aileron Aileron hinge Wing butt rib (or compression rib or bulkhead rib)
Figure 1-32. Basic wood wing structure and components.
1-16

Figure 1-33. Wing root fairings smooth airflow and hide wing
attach fittings.
of the wing tips to damage,especially during groundhandling
and taxiing. Figure 1-34 shows a removable wing tip for a
large aircraft wing. Others are different. The wing tip
assembly is ofaluminumalloy construction.The wing tip cap
is securedto the tip with countersunkscrews and is securedto
the interspar structure at four points with ¼-inch diameter
bolts.To preventice fromforming on the leading edge of the
wings of large aircraft, hot air from an engine is often
channeled through the leading edge fromwing root to wing
tip. A louver on the top surface of the wing tip allows this
warm air to be exhausted overboard.Wing position lights are
located at the center of the tip and are not directly visible
from the cockpit. As an indication that the wing tip light is
operating, some wing tips are equipped with a Lucite rod to
transmit the light to the leading edge.
covered with a fairing to achieve smooth airflow in this
area. The fairing(s) can be removed for access to the wing
attach fittings. [Figure 1-33]
The wing tip is often a removable unit,bolted to the outboard
end ofthe wing panel.One reason forthis is the vulnerability
Wing Skin
Often,the skin on a wing is designedto carry part ofthe flight
and ground loads in combination with the spars andribs.This
is known as a stressed-skin design. The all-metal, full
cantileverwing section illustrated in Figure 1-35 shows the
structure of one such design. The lack of extra internal
Accesspanel Upper skin
Points of attachment to front and rear
spar fittings (2 upper, 2 low er)
Louver
Wing tip navigation light
Leading edge outer skin
Corrugated inner skin
Reflector rod Heat duct Wing cap
Figure 1-34. Aremovable metal wing tip.
1-17

Figure 1-35. Theskin is an integral load carrying part of a stressed skin design.
or external bracing requires that the skin share some of the
load. Notice the skin is stiffened to aid with this function.
Fuel is often carried inside the wings of a stressed-skin
aircraft. The joints in the wing can be sealed with a special
fuel resistantsealant enablingfuelto be stored directly inside
the structure.This is known as wet wing design. Alternately,
a fuel-carrying bladder or tank can be fitted inside a wing.
Figure 1-36 showsa wing sectionwith a boxbeamstructural
design such asone that might be found in a transport category
aircraft. This structure increases strength while reducing
weight.Propersealing ofthe structure allows fuelto be stored
in the boxsections of the wing.
The wing skin on an aircraft may be made from a wide variety of
materials such as fabric, wood, or aluminum. But a single thin
sheet of material is not always employed. Chemically
milled aluminum skin can provide skin of varied
thicknesses. On aircraft with stressed-skin wing design,
honeycomb structured wing panels are often used as skin.
A honeycomb structure is built up from a core material
resembling a bee hive’s honeycomb which is laminated or
sandwiched between thin outer skin sheets. Figure 1-37
illustrates honeycomb panes and their components. Panels
formed like this are lightweight and very strong. They
have a variety of uses on the aircraft, such as floor panels,
bulkheads, and control surfaces, as well as wing skin
panels. Figure 1-38 shows the locations of honeycomb
construction wing panels on a jet transport aircraft.
A honeycomb panel can be made from a wide variety of
materials. Aluminum core honeycomb with an outer skin of
aluminum is common. But honeycomb in which the core is
Sealed structure fueltank—wet wing
Figure 1-36. Fuel is often carried in the wings.
1-18

an Arimid® fiber and the outersheets are coatedPhenolic® is
common as well. In fact, a myriad of other material
combinations such as those using fiberglass, plastic,
Nomex®, Kevlar®, and carbon fiber all exist. Each
honeycomb structure possesses unique characteristics
depending uponthe materials,dimensions,andmanufacturing
techniques employed. Figure 1-39 shows an entire wing
leading edge formed fromhoneycomb structure.
Nacelles
Nacelles (sometimes called “pods”) are streamlined
enclosures used primarily to house the engine and its
components.They usually presenta round or elliptical profile
to the wind thus reducing aerodynamic drag. On most single-
engine aircraft,the engine and nacelle are at the forward end
of the fuselage. On multiengine aircraft, engine nacelles are
built into the wings or attached to the fuselage
at the empennage (tail section). Occasionally, a multiengine
aircraft is designed with a nacelle in line with the fuselage aft
of the passenger compartment. Regardless of its location, a
nacelle contains the engine and accessories, engine mounts,
structural members, a firewall, and skin and cowling on the
exterior to fare the nacelle to the wind.
Some aircraft have nacelles that are designed to house the
landing gear when retracted. Retracting the gear to reduce
wind resistance is standard procedure on high-
performance/ high-speed aircraft. The wheel well is the
area where the landing gear is attached and stowed when
retracted. Wheel wells can be located in the wings and/or
fuselage when not part of the nacelle. Figure 1-40 shows
an engine nacelle incorporating the landing gear with the
wheel well extending into the wing root.
Core Skin
A
Skin
Constant thickness
Core
B
Skin
Skin
Tapered core
Figure 1-37. Thehoneycomb panel is a staple in aircraft construction. Cores can be either constant thickness (A) or tapered (B).
Tapered core honeycomb panels are frequently used as flight control surfaces and wing trailing edges.
1-19

Trailing edge sandw ich panels
constant-thicknesscore
Wing leading edge
Spoiler sandw ich panel
tapered core, solid w edge
Trailing edge sandw ich panels
Outboard flap
Aileron tab sandw ichpanel
tapered core, Phenolic w edge
Inboard flap
Spoiler sandw ich panel
tapered core, solid w edge
Aileron tab sandw ichpanel
Trailing edge w edge sandwich panel
tapered core, cord w edge
Figure 1-38. Honeycomb wing construction on a large jet transport aircraft.
The framework of a nacelle usually consists of structural
members similar to those of the fuselage. Lengthwise
members, such as longerons and stringers, combine with
horizontal/vertical members, such as rings, formers, and
bulkheads, to give the nacelle its shape and structural
integrity. A firewall is incorporated to isolate the engine
compartment from the rest of the aircraft. This is basically
a stainless steel or titanium bulkhead that contains a fire in
the confines of the nacelle rather than letting it spread
throughout the airframe. [Figure 1-41]
Engine mounts are also found in the nacelle. These are the
structural assemblies to which the engine is fastened. They
are usually constructed from chrome/molybdenum steel
tubing in light aircraft and forged chrome/nickel/
molybdenum assemblies in larger aircraft. [Figure 1-42]
The exterior of a nacelle is covered with a skin orfitted with a
cowling which can be opened to access the engine and
1-20
components inside. Both are usually made of sheet
aluminum or magnesium alloy with stainless steel or
titanium alloys being used in high-temperature areas, such
as around the exhaust exit. Regardless of the material used,
the skin is typically attached to the framework with rivets.
Cowling refers to the detachable panels covering those areas
into which access must be gained regularly, such as the
engine and its accessories.It is designed to provide a smooth
airflow over the nacelle and to protect the engine from
damage. Cowl panels are generally made of aluminumalloy
construction. However, stainless steel is often used as the
innerskin aft of the powersectionand forcowlflaps and near
cowl flap openings. It is also used for oil cooler ducts. Cowl
flaps are moveable parts ofthe nacelle cowling that open and
close to regulate engine temperature.
There are many engine cowl designs. Figure 1-43 shows an
exploded view of the pieces of cowling for a horizontally

Metal w ing spar
Metal member bonded to sandwich
Honeycomb sandw ich core Wooden members spanwise and chordwise
Glass reinforced plastics sandwich the core
Figure 1-39. Awing leading edge formed from honeycomb material bonded to the aluminum spar structure.
Figure 1-40. Engine nacelle incorporating the landing gear with the wheel well extending into the wing root.
1-21

Figure 1-41. An engine nacelle firewall.
opposedengineon a light aircraft.It is attachedto the nacelle
by means of screws and/or quick release fasteners. Some
large reciprocating engines are enclosed by “orange peel”
cowlings which provide excellent access to components
inside the nacelle. [Figure 1-44] These cowl panels are
attached to the forward firewall by mounts which also serve
as hinges for opening the cowl. The lower cowl mounts are
secured to the hinge bracketsby quick release pins. The side
and top panels are held open by rods and the lower panel is
retained in the open position by a spring and a cable. All of
the cowling panels are locked in the closed position by over-
centersteellatches which are secured in the closed position
by spring-loaded safety catches.
An example of a turbojet engine nacelle can be seen in
Figure 1-45. The cowl panels are a combination of fixed
and easily removable panels which can be opened and
closed during maintenance. A nose cowl is also a feature
on a jet engine nacelle. It guides air into the engine.
Empennage
The empennage of an aircraft is also known as the tail
section. Most empennage designs consist of a tail cone,
fixed aerodynamic surfaces or stabilizers, and movable
aerodynamic surfaces.
Figure 1-42. Various aircraft engine mounts.
Figure 1-43. Typical cowling for a horizontally opposed
reciprocating engine.
The tail cone serves to close and streamline the aft end of
most fuselages. The cone is made up of structural
members like those of the fuselage; however, cones are
usually of lighter construction- since they receive less
stress than the fuselage. [Figure 1-46]
1-22

Figure 1-44. Orange peel cowling for large radial reciprocating engine.
Figure 1-45. Cowling on a transportcategory turbine engine nacelle.
1-23

Frame Longeron
Skin
Stringer Bulkhead
Figure 1-46. The fuselage terminates at the tail cone with similar
but more lightweight construction.
The other components of the typical empennage are of
heavier construction than the tail cone. These members
include fixed surfaces that help stabilize the aircraft and
movable surfaces that help to direct an aircraft during flight.
The fixed surfaces are the horizontal- stabilizer and vertical
stabilizer. The movable surfacesare usually a rudder located
at the aft edge ofthe verticalstabilizer and an elevatorlocated
at the aft edge the horizontal stabilizer. [Figure 1-47]
The structure ofthe stabilizers is very similar to that which is
used in wing construction. Figure 1-48 shows a typical
verticalstabilizer. Notice the use ofspars,ribs, stringers, and
skin like those found in a wing. They perform the same
functions shaping and supporting the stabilizer and
transferring stresses. Bending, torsion, and shear created by
air loads in flight pass fromone structuralmemberto another.
Each member absorbs some of the stress and passes
Verticalstabilizer
Horizontal stabilizer Rudder
Trim tabs
Elevator
Figure 1-47. Components of a typical empennage.
Stringer
Rib
Spars Skin
Figure 1-48. Vertical stabilizer.
the remainder on to the others. Ultimately, the spar
transmits any overloads to the fuselage. A horizontal
stabilizer is built the same way.
The rudder and elevator are flight control surfaces that are
also part of the empennage discussed in the next section of
this chapter.
Flight Control Surfaces
The directional control of a fixed-wing aircraft takes place
around the lateral,longitudinal,and verticalaxes by means of
flight control surfaces designed to create movement about
these axes. These control devices are hinged or movable
surfaces throughwhich the attitude ofan aircraft is controlled
during takeoff, flight, and landing. They are usually divided
into two major groups: 1) primary- or main flight control
surfaces and 2) secondary or auxiliary control surfaces.
Primary Flight Control Surfaces
The primary flight control surfaces on a fixed-wing
aircraft include: ailerons, elevators, and the rudder. The
ailerons are attached to the trailing edge of both wings and
when moved, rotate the aircraft around the longitudinal
axis. The elevator is attached to the trailing edge of the
horizontal stabilizer. When it is moved, it alters aircraft
pitch, which is the attitude about the horizontal or lateral
axis. The rudder is hinged to the trailing edge of the
vertical stabilizer. When the rudder changes position, the
aircraft rotates about the vertical axis (yaw). Figure 1-49
shows the primary flight controls of a light aircraft and the
movement they create relative to the three axes of flight.
1-24

Elevator
Lateral
—Pitch
stability)(longitudinalaxis
Rudder—Yaw
Verticalaxis
(directional
stability)
—Roll
Aileron
Longitudinal
axis (lateral
stability)
Primary control surfaces constructed from composite
materials are also commonly used.These are found on many
heavy and high-performance aircraft, as well as gliders,
home-built,and light-sport aircraft. The weight and strength
advantages overtraditionalconstructioncan be significant. A
wide variety of materials and construction techniques are
employed. Figure 1-51 shows examples of aircraft that use
composite technology on primary flight controlsurfaces.Note
that the controlsurfaces offabric-covered aircraft often have
fabric-covered surfaces just as aluminum-skinned (light)
aircraft typically have all-aluminumcontrolsurfaces.There is
a critical need forprimary controlsurfaces to be balanced so
they do not vibrate or flutter in the wind.
Primary
Airplane Axes of Type of
Control
Movement Rotation Stability
Surface
Aileron Roll Longitudinal Lateral
Elevator/
Pitch Lateral Longitudinal
Stabilator
Rudder Yaw Vertical Directional
Figure 1-49. Flight control surfaces move the aircraft around the
three axes of flight.
Primary control surfaces are usually similar in construction-
to one another and vary only in size, shape, and methods of
attachment.On aluminumlight aircraft,their structure is often
similar to an all-metal wing. This is appropriate because the
primary control surfaces are simply smaller aerodynamic
devices. They are typically made from an aluminum alloy
structure built arounda single-sparmemberortorque tube to
which ribs are fitted and a skin is attached. The lightweight
ribs are, in many cases,stamped out fromflat aluminum sheet
stock. Holes in the ribs lighten the assembly. An aluminum
skin is attachedwith rivets. Figure 1-50 illustrates this typeof
structure,which can be foundon the primary controlsurfaces
of light aircraft as well as on mediumand heavy aircraft.
Aileron hinge-pin fitting
Actuating horn
Spar Lightning hole
Figure 1-50. Typical structure of an aluminum flight control surface.
Figure 1-51. Compositecontrol surfaces and some of the many
aircraft that utilize them.
1-25

Performed to manufacturer’s instructions, balancing usually
consistsof assuring that the center of gravity of a particular
device is at or forward of the hinge point. Failure to properly
balance a control surface could lead to catastrophic failure.
Figure 1-52 illustrates several aileron configurations with
their hinge points well aft of the leading edge. This is a
common design feature used to prevent flutter.
Ailerons
Ailerons are the primary flight control surfaces that move
the aircraft about the longitudinal axis. In other words,
movement of the ailerons in flight causes the aircraft to
roll. Ailerons are usually located on the outboard trailing
edge of each of the wings. They are built into the wing and
are calculated as part of the wing’s surface area. Figure 1-
53 shows aileron locations on various wing tip designs.
Ailerons are controlled by a side-to-side motion ofthe control
stickin the cockpit ora rotation ofthe controlyoke.Whenthe
aileron on one wing deflects down,the aileron on the opposite
wing deflects upward. This amplifies the movement of the
aircraft around the longitudinal axis. On the wing on which
the aileron trailing edge moves downward, camber is
increased,and lift is increased.Conversely,on the otherwing,
the raised aileron decreases lift. [Figure 1-54] The result is
Up aileron
Dow n aileron
Figure 1-54. Differential aileroncontrol movement.When one aileron is
moved down, the aileron on the opposite wing is deflected upward.
a sensitive response to the control input to roll the aircraft.
The pilot’s request for aileron movement and roll are
transmitted fromthe cockpit to the actualcontrol surface in a
variety ofways dependingon the aircraft.A systemofcontrol
cables and pulleys,push-pull tubes, hydraulics, electric, or a
combination of these can be employed. [Figure 1-55]
Simple, light aircraft usually do not havehydraulic or electric
fly-by-wire aileron control. These are found on heavy and
high-performance aircraft. Large aircraft and some high-
performance aircraft may also have a second set of ailerons
located inboard on the trailing edge of the wings. These are
part of a complex system of primary and secondary control
surfaces usedto provide lateralcontrol and stability in flight.
At low speeds,the ailerons may be augmented by the use of
flaps and spoilers. At high speeds, only inboard aileron
deflection is required to roll the aircraft while the
Figure 1-52. Aileron hinge locations are veryclose to but aft of
the center of gravity to prevent flutter.
Stop
Elevator cables
Tether stop
Stop
To ailerons
Note pivots not on center of shaft
Figure 1-55. Transferring control surface inputs from the cockpit.
Figure 1-53. Aileron location on various wings.
1-26

other control surfaces are locked out or remain stationary.
Figure 1-56 illustrates the location of the typical flight
control surfaces found on a transport category aircraft.
Elevator
The elevator is the primary flight control surface that moves the
aircraft around the horizontal or lateral axis. This causes the nose
of the aircraft to pitch up or down. The elevator is hinged to the
trailing edge of the horizontal stabilizer and typically spans most
or all of its width. It is controlled in the cockpit by pushing or
pulling the control stick or yoke forward or aft.
Light aircraft use a system of control cables and pulleys or
push-pull tubes to transfer cockpit inputs to the movement
of the elevator. High-performance and large aircraft
typically employ more complex systems. Hydraulic power
is commonly used to move the elevator on these aircraft.
On aircraft equipped with fly-by-wire controls, a
combination of electrical and hydraulic power is used.
Rudder
The rudder is the primary control surface that causes an
aircraft to yawor move about the vertical axis. This provides
directionalcontroland thus points the nose of the aircraft in
the direction desired. Most aircraft have a single rudder
hinged to the trailing edge of the vertical stabilizer. It is
controlled by a pair of foot-operated rudder pedals in the
cockpit. When the right pedal is pushed forward, it deflects
Inboard aileron
the rudder to the right which moves the nose of the aircraft
to the right. The left pedal is rigged to simultaneously
move aft. When the left pedal is pushed forward, the nose
of the aircraft moves to the left.
As with the other primary flight controls, the transfer of the
movement ofthe cockpit controls to the ruddervaries with the
complexity of the aircraft. Many aircraft incorporate the
directionalmovement ofthe nose ortailwheelinto the rudder
controlsystemforground operation.This allows the operator
to steer the aircraft with the rudder pedals during taxi when
the airspeed is not high enoughforthe control surfaces to be
effective.Some large aircraft have a split rudderarrangement.
This is actually two rudders, one above the other. At low
speeds, both rudders deflect in the same direction when the
pedals are pushed. At higher speeds, one of the rudders
becomes inoperative as the deflection of a single rudder is
aerodynamically sufficient to maneuver the aircraft.
Dual Purpose Flight Control Surfaces
The ailerons, elevators, and rudder are considered
conventional primary control surfaces. However, some
aircraft are designed with a control surface that may serve a
dual purpose. For example, elevons perform the combined
functions of the ailerons and the elevator. [Figure 1-57]
A movable horizontal tail section, called a stabilator, is a
controlsurface that combines theaction ofboth the horizontal
Flight spoilers
Outboard aileron
Figure 1-56. Typical flight control surfaces on a transport category aircraft.
1-27

Elevons
Figure 1-57. Elevons.
Figure 1-58. A stabilizer and index marks on a transport
category aircraft.
stabilizer and the elevator. [Figure 1-58] Basically, a
stabilator is a horizontal stabilizer that can also be rotated
about the horizontal axis to affect the pitch of the aircraft.
A ruddervatorcombinesthe actionofthe rudderand elevator.
[Figure 1-59] This is possible on aircraft with V–tail
empennages where the traditional horizontal and vertical
stabilizers do not exist.Instead,two stabilizers angle upward
and outward fromthe aft fuselage in a “V” configuration.
Ruddervator
Figure 1-59. Ruddervator.
Each contains a movable ruddervator built into the trailing
edge. Movement of the ruddervators can alter the
movement of the aircraft around the horizontal and/or
vertical axis. Additionally, some aircraft are equipped with
flaperons. [Figure 1-60] Flaperons are ailerons which can
also act as flaps. Flaps are secondary control surfaces on
most wings, discussed in the next section of this chapter.
Secondary or Auxiliary Control Surfaces
There are several secondary or auxiliary flight control
surfaces. Their names, locations, and functions of those for
most large aircraft are listed in Figure 1-61.
Flaps
Flaps are found on most aircraft.They are usually inboard on
the wings’ trailing edges adjacent to the fuselage. Leading
edge flaps are also common. They extend forward and down
from the inboard wing leading edge.The flaps are lowered to
increase the camberofthe wings and provide greater lift and
controlat slowspeeds.They enable landing at slower speeds
and shorten the amount of runway required for takeoff and
landing.The amount that the flaps extend and the angle they
form with the wing can be selected from the cockpit.
Typically,flaps can extend up to 45–50°. Figure 1-62 shows
various aircraft with flaps in the extended position.
Flaps are usually constructed of materials and with
techniques used on the other airfoils and control surfaces
of a particular aircraft. Aluminum skin and structure flaps
are the norm on light aircraft. Heavy and high-performance
aircraft flaps may also be aluminum, but the use of
composite structures is also common.
There are various kinds of flaps. Plain flaps formthe trailing
edge of the wing when the flap is in the retracted position.
[Figure 1-63A] The airflowoverthe wing continues over the
upperand lowersurfaces ofthe flap, making the trailing edge
Flaperons
Figure 1-60. Flaperons.
1-28

Secondary/Auxiliary Flight Control Surfaces
Name Location Function
Flaps Inboard trailing edge of wings
Extends the camber of the wing for greater lift and slower flight.
Allows control at low speeds for short field takeoffs and landings.
Trailing edge of primary
Trim tabs Reduces the force needed to move a primary control surface.
flight control surfaces
Balance tabs Reduces the force needed to move a primary control surface.
Anti-balance tabs Increases feel and effectiveness of primary control surface.
Servo tabs Assists or provides the force for moving a primary flight control.
Spoilers Upper and/or trailing edge of wing Decreases (spoils) lift. Can augment aileron function.
Slats Mid to outboard leading edge of wing
Slots
Outer leading edge of wing Directs air over upper surface of wing during high angle of attack.
forward of ailerons Lowers stall speed and provides control during slow flight.
Leading edge flap Inboard leading edge of wing
NOTE: An aircraft may possessnone, one, or a combination of the above controlsurfaces.
Figure 1-61. Secondary or auxiliary control surfaces and respective locations for larger aircraft.
Figure 1-62. Various aircraft with flaps in the extended position.
A
Plain flap
B
Split flap
C
Fow ler flap
Figure 1-63. Various types of flaps.
1-29

of the flap essentially the trailing edge of the wing. The plain
flap is hinged so that the trailing edge can be lowered. This
increases wing camber and provides greater lift.
A split flap is normally housed under the trailing edge of the
wing. [Figure 1-63B] It is usually just a braced flat metal
plate hinged at several places along its leading edge. The
uppersurface of the wing extends to the trailing edge of the
flap. When deployed,the split flap trailing edge lowers away
from the trailing edge ofthe wing.Airflow overthe top ofthe
wing remains the same. Airflow under the wing now follows
the camber created by the lowered split flap, increasing lift.
Fowler flaps not only lower the trailing edge of the wing when
deployed but also slide aft, effectively increasing the area of the
wing. [Figure 1-63C] This creates more lift via the increased
surface area, as well as the wing camber. When stowed, the
fowler flap typically retracts up under the wing trailing edge
similar to a split flap. The sliding motion of a fowler flap can be
accomplished with a worm drive and flap tracks.
An enhanced version of the fowler flap is a set of flaps that
actually containsmore than oneaerodynamic surface. Figure
1-64 showsa triple-slotted flap.In this configuration, the flap
consists of a fore flap, a mid flap, and an aft flap. When
deployed, each flap section slides aft on tracks as it lowers.
The flap sectionsalsoseparate leaving an open slot between
the wing and the fore flap, as well as between eachofthe flap
sections. Air from the underside of the wing flows through
these slots. The result is that the laminar flow on the upper
surfaces is enhanced. The greater camber and effective wing
area increase overall lift.
Heavy aircraft often have leading edge flaps that are used in
conjunction with the trailing edge flaps. [Figure 1-65] They
can be made of machined magnesium or can have an
aluminum orcomposite structure.While theyare not installed
or operate independently, their use with trailing edge flaps
can greatly increase wing camber and lift. When stowed,
leading edge flaps retract into the leading edge of the wing.
Retracted
Fore flap
Mid flap
Aft flap
Figure 1-64. Triple-slotted flap.
Hinge point
Actuator
Flap extended
Flap retracted
Retractable nose
Figure 1-65. Leading edge flaps.
The differing designs of leading edge flaps essentially
provide the same effect. Activation of the trailing edge
flaps automatically deploys the leading edge flaps, which
are driven out of the leading edge and downward,
extending the camber of the wing. Figure 1-66 shows a
Krueger flap, recognizable by its flat mid-section.
Slats
Another leading edge device which extends wing camber
is a slat. Slats can be operated independently of the flaps
with their own switch in the cockpit. Slats not only extend
out of the leading edge of the wing increasing camber and
lift, but most often, when fully deployed leave a slot
between their trailing edges and the leading edge of the
wing. [Figure 1-67] This increases the angle of attack at
which the wing will maintain its laminar airflow, resulting
in the ability to fly the aircraft slower with a reduced stall
speed, and still maintain control.
Spoilers and Speed Brakes
A spoiler is a device found on the upper surface of many
heavy and high-performance aircraft. It is stowed flush to
the wing’s upper surface. When deployed, it raises up into
the airstream and disrupts the laminar airflow of the wing,
thus reducing lift.
Spoilers are made with similar construction materials and
techniques as the otherflight control surfaces on the aircraft.
Often, they are honeycomb-core flat panels. At low speeds,
spoilers are rigged to operate when the ailerons operate to
assist with the lateral movement and stability of the aircraft.
On the wing where the aileron is moved up, the spoilers also
raise thus amplifying the reduction of lift on that wing.
[Figure 1-68] On the wing with downward aileron deflection,
1-30

Figure 1-66. Side view (left) and front view (right) of a Krueger flap on a Boeing 737.
Figure 1-67. Air passing through the slot aft of the slat promotes
boundary layer airflow on the upper surface at high angles of attack.
Figure 1-68. Spoilers deployed upon landing on a transport
category aircraft.
the spoilers remain stowed. As the speed of the aircraft
increases, the ailerons become more effective and the
spoiler interconnect disengages.
Spoilers are unique in that they may also be fully deployed on
both wings to act as speed brakes. The reduced lift and
increased drag canquickly reduce the speed of the aircraft in
flight. Dedicated speedbrake panels similar to flight spoilers
in construction can alsobe foundon the upper surface of the
wings of heavy and high-performance aircraft. They are
designedspecifically to increasedrag and reduce the speed of
the aircraft when deployed.These speed brake panels do not
operate differentially with the ailerons at low speed.
The speed brake control in the cockpit can deploy all spoiler
and speed brake surfaces fully when operated. Often, these
surfaces are also riggedto deployon the ground automatically
when engine thrust reversers are activated.
Tabs
The force of the air against a control surface during the high
speed of flight can make it difficult to move and hold that control
surface in the deflected position. A control surface might also be
too sensitive for similar reasons. Several different tabs are used
to aid with these types of problems. The table in Figure 1-69
summarizes the various tabs and their uses.
1-31

Flight Control Tabs
Type
Direction of Motion
Activation Effect
(in relation to control surface)
Trim Opposite Set by pilot from cockpit.
Statically balances the aircraft
in flight. Allow s “hands off”
Uses independent linkage. maintenance of flight condition.
Moves when pilot moves control surface.
Balance Opposite
Aids pilot in overcoming the force
Coupled to control surface linkage. needed to move the controlsurface.
Directly linked to flight control
Servo Opposite
Aerodynamically positions control
input device. Can be primary surfaces that require too much
or back-up means of control. force to move manually.
Anti-balance
Same
Directly linked to flight
Increases force needed by pilot
to change flight control position.
or Anti-servo control input device. De-sensitizes flight controls.
Spring Opposite
Located in line of direct linkage to servo Enables moving control surface
tab. Spring assists when control forces when forces are high.
become too high in high-speed flight. Inactive during slow flight.
Figure 1-69. Various tabs and their uses.
While in flight, it is desirable for the pilot to be able to take his or
her hands and feet off of the controls and have the aircraft
maintain its flight condition. Trims tabs are designed to allow
this. Most trim tabs are small movable surfaces located on the
trailing edge of a primary flight control surface. A small
movement of the tab in the direction opposite of the direction the
flight control surface is deflected, causing air to strike the tab, in
turn producing a force that aids in maintaining the flight control
surface in the desired position. Through linkage set from the
cockpit, the tab can be positioned so that it is actually holding the
control surface in position rather than the pilot. Therefore,
elevator tabs are used to maintain the speed of the aircraft since
they assist in maintaining the selected pitch. Rudder tabs can be
set to hold yaw in check and maintain heading. Aileron tabs can
help keep the wings level.
Occasionally, a simple light aircraft may have a stationary
metal plate attached to the trailing edge of a primary flight
control,usually the rudder.This is also a trimtab as shown in
Figure 1-70.It can be bent slightly on the ground to trimthe
aircraft in flight to a hands-offcondition when flying straight
and level.The correct amount of bend can be determined only
by flying the aircraft after an adjustment. Note that a small
amount of bending is usually sufficient.
The aerodynamic phenomenon of moving a trim tab in one
direction to cause the control surface to experience a force
moving in the opposite direction is exactly what occurs with the
use of balance tabs. [Figure 1-71] Often, it is difficult to move a
primary control surface due to its surface area and the speed of
the air rushing over it. Deflecting a balance tab hinged at the
trailing edge of the control surface in the opposite
1-32
Ground adjustable rudder trim
Figure 1-70. Example of a trim tab.
direction of the desired control surface movement causes a force
to position the surface in the proper direction with reduced force
to do so. Balance tabs are usually linked directly to the control
surface linkage so that they move automatically when there is an
input for control surface movement. They also can double as trim
tabs, if adjustable in the flight deck.
A servo tab is similar to a balance tab in location and
effect, but it is designed to operate the primary flight
control surface, not just reduce the force needed to do so.
It is usually used as a means to back up the primary
control of the flight control surfaces. [Figure 1-72]

Lift
Tab geared to deflect proportionally to the
control deflection, but in the opposite direction
Fixed surface
Tab
surfaces are signaled by electric input. In the case of
hydraulic system failure(s), manual linkage to a servo tab
can be used to deflect it. This, in turn, provides an
aerodynamic force that moves the primary control surface.
A controlsurface may require excessive force to move only in
the final stages of travel. When this is the case, a spring tab
can be used. This is essentially a servo tab that does not
activate until an effort is made to move the control surface
beyonda certain point.When reached, a spring in line of the
controllinkage aids in moving the controlsurfacethroughthe
remainder of its travel. [Figure 1-73]
Figure 1-71. Balance tabs assist with forces needed to position
control surfaces.
Figure 1-74 shows anotherway ofassisting the movement of
an aileron on a large aircraft. It is called an aileron balance
panel. Not visible when approaching the aircraft, it is
positioned in the linkage that hinges the aileron to the wing.
Control stick
Free link
Control surface hinge line
Figure 1-72. Servo tabs can be used to position flight control
surfaces in case of hydraulic failure.
Balance panels have been constructed typically of aluminum
skin-covered frame assemblies or aluminum honeycomb
structures. The trailing edge of the wing just forward of the
leading edge of the aileron is sealed to allow controlled airflow in
and out of the hinge area where the balance panel is located.
Control stick
Spring
Free link
On heavy aircraft, large control surfaces require too much
force to be moved manually and are usually deflected out
of the neutral position by hydraulic actuators. These power
control units are signaled via a system of hydraulic valves
connected to the yoke and rudder pedals. On fly-by-wire
aircraft, the hydraulic actuators that move the flight control
Figure 1-73. Many tab linkages have a spring tab that kicks in as
the forces needed to deflect a control increase with speed and the
angle of desired deflection.
Hinge
Balance panel Vent gap Control tab
WING
AILERON
Vent gap Low er pressure
Figure 1-74. An aileron balance panel and linkage uses varying air pressure to assist in control surface positioning.
1-33

[Figure 1-75] When the aileron is moved from the neutral
position, differential pressure builds up on one side of the
balance panel. This differential pressure acts on the balance panel
in a direction that assists the aileron movement. For slight
movements, deflecting the control tab at the trailing edge of the
aileron is easy enough to not require significant assistance from
the balance tab. (Moving the control tab moves the ailerons as
desired.) But, as greater deflection is requested, the force
resisting control tab and aileron movement becomes greater and
augmentation from the balance tab is needed. The seals and
mounting geometry allow the differential pressure of airflow on
the balance panel to increase as deflection of the ailerons is
increased. This makes the resistance felt when moving the aileron
controls relatively constant.
Antiservo tabs, as the name suggests, are like servo tabs
but move in the same direction as the primary control
surface. On some aircraft, especially those with a movable
horizontal stabilizer, the input to the control surface can be
too sensitive. An antiservo tab tied through the control
linkage creates an aerodynamic force that increases the
effort needed to move the control surface. This makes
flying the aircraft more stable for the pilot. Figure 1-76
shows an antiservo tab in the near neutral position.
Deflected in the same direction as the desired stabilator
movement, it increases the required control surface input.
Other Wing Features
There may be other structures visible on the wings of an
aircraft that contribute to performance. Winglets, vortex
generators, stall fences, and gap seals are all common wing
features. Introductory descriptions of each are given in the
following paragraphs.
A winglet is an obvious vertical upturn of the wing’s tip
resembling a vertical stabilizer. It is an aerodynamic
device designed to reduce the drag created by wing tip
vortices in flight. Usually made from aluminum or
composite materials, winglets can be designed to optimize
performance at a desired speed. [Figure 1-77]
Antiservo tab
Stabilator pivot point
Figure 1-76. An antiservo tab moves in the same direction as the
control tab. Shown here on a stabilator, it desensitizes the pitch
control.
Vortex generators are small airfoil sections usually attached
to the upper surface of a wing. [Figure 1-78] They are
designed to promote positive laminar airflow over the wing
and controlsurfaces.Usually made ofaluminum and installed
in a spanwise line or lines, the vortices created by these
devices swirl downward assisting maintenance of the
boundary layerofair flowing overthe wing.They can also be
found on the fuselage andempennage. Figure 1-79shows the
unique vortexgenerators on a Symphony SA-160 wing.
A chordwise barrier on the upper surface of the wing,
called a stall fence, is used to halt the spanwise flow of air.
During low speed flight, this can maintain proper
chordwise airflow reducing the tendency for the wing to
stall. Usually made of aluminum, the fence is a fixed
structure most common on swept wings, which have a
natural spanwise tending boundary air flow. [Figure 1-80]
Balance panel
Figure 1-75. Thetrailing edge of the wing just forward of the leading
edge of the aileron is sealed to allow controlled airflow in and out Figure 1-77. Awinglet reduces aerodynamic drag caused by air
of the hinge area where the balance panel is located. spilling off of the wing tip.
1-34

Stall fence
Figure 1-78. Vortex generators.
Figure 1-80. Astall fence aids in maintaining chordwise airflow
over the wing.
Figure 1-79. The Symphony SA-160 has two unique vortex
generators on its wing to ensure aileron effectiveness through the
stall.
Often,a gap can exist between the stationary trailing edge of
a wing or stabilizer and the movable control surface(s). At
high angles of attack, high pressure air fromthe lower wing
surface can be disrupted at this gap. The result can be
turbulent airflow, which increases drag. There is also a
tendency for some lower wing boundary air to enter the gap
and disrupt the upper wing surface airflow, which in turn
reduces lift and control surface responsiveness. The use of
gap seals is common to promote smooth airflow in these
gap areas. Gap seals can be made of a wide variety of
materials ranging from aluminum and impregnated fabric
to foam and plastic. Figure 1-81 shows some gap seals
installed on various aircraft.
Landing Gear
The landing gear supports the aircraft during landing and while it
is on the ground. Simple aircraft that fly at low speeds generally
have fixed gear. This means the gear is stationary and does not
retract for flight. Faster, more complex aircraft have retractable
landing gear. After takeoff, the landing gear is retracted into the
fuselage or wings and out of the airstream. This is important
because extended gear create significant parasite drag which
reduces performance. Parasite drag is caused by the friction of
the air flowing over the
Aileron gap seal
Tab gap seal
Figure 1-81. Gap seals promote the smooth flow of air over gaps between fixed and movablesurfaces.
1-35

gear. It increases with speed. On very light, slow aircraft,
the extra weight that accompanies a retractable landing
gear is more of a detriment than the drag caused by the
fixed gear. Lightweight fairings and wheel pants can be
used to keep drag to a minimum. Figure 1-82 shows
examples of fixed and retractable gear.
Landing gear must be strong enough to withstand the forces of
landing when the aircraft is fully loaded. In addition to strength, a
major design goal is to have the gear assembly be as light as
possible. To accomplish this, landing gear are made from a wide
range of materials including steel, aluminum, and magnesium.
Wheels and tires are designed specifically for aviation use and
have unique operating characteristics. Main wheel assemblies
usually have a braking system. To aid with the potentially high
impact of landing, most landing gear have a means of either
absorbing shock or accepting shock and distributing it so that the
structure is not damaged.
Not all aircraft landing gear are configured with wheels.
Helicopters, for example, have such high maneuverability
and low landing speeds that a set of fixed skids is common
and quite functional with lower maintenance. The same is
true for free balloons which fly slowly and land on wood
skids affixed to the floor of the gondola. Other aircraft
landing gear are equipped with pontoons or floats for
operation on water. A large amount of drag accompanies
this type of gear, but an aircraft that can land and take off
on water can be very useful in certain environments. Even
skis can be found under some aircraft for operation on
snow and ice. Figure 1-83 shows some of these alternative
landing gear, the majority of which are the fixed gear type.
Figure 1-83. Aircraft landing gear without wheels.
Figure 1-82. Landing gear can be fixed (top) or retractable (bottom).
Amphibious aircraft are aircraft than can land either on land
or on water. On some aircraft designed for such dual usage,
the bottom half of the fuselage acts as a hull. Usually, it is
accompanied by outriggers on the undersideofthe wings near
the tips to aid in water landing and taxi. Main gear that
1-36

retract into the fuselage are only extended when landing on
the ground or a runway. This type of amphibious aircraft is
sometimes called a flying boat. [Figure 1-84]
Many aircraft originally designed for land use can be fitted
with floats with retractable wheels for amphibious use.
[Figure 1-85]Typically,the gearretracts into the float when
not needed. Sometimes a dorsal fin is added to the aft
underside of the fuselage for longitudinal stability during
wateroperations.It is even possible on some aircraft to direct
this type of fin by tying its control into the aircraft’s rudder
pedals.Skis can also be fitted with wheels that retract to allow
landing on solid ground or on snow and ice.
Tail Wheel Gear Configuration
There are two basic configurations of airplane landing gear:
conventionalgearortail wheelgearand the tricycle gear. Tail
wheel gear dominated early aviation and therefore has
become known as conventional gear. In addition to its two
main wheels which are positioned undermost ofthe weight of
the aircraft, the conventional gear aircraft also has a smaller
wheel located at the aft end of the fuselage.
Figure 1-84. An amphibious aircraft is sometimes called a flying
boat because the fuselage doubles as a hull.
Figure 1-85. Retractable wheels makethis aircraft amphibious.
Figure 1-86. An aircraft with tail wheel gear.
[Figure 1-86] Often this tail wheel is able to be steered by
rigging cables attached to the rudder pedals. Other
conventional gear have no tail wheel at all using just a steel
skid plate underthe aft fuselage instead. The small tail wheel
or skid plate allows the fuselage to incline, thus giving
clearance for the long propellers that prevailed in aviation
through WWII. It also gives greater clearance between the
propeller and loose debris when operating on an unpaved
runway. But the inclined fuselage blocks the straight-ahead
vision ofthe pilot during groundoperations.Untilup to speed
where the elevatorbecomes effective to lift the tail wheel off
the ground, the pilot must lean his head out the side of the
cockpit to see directly ahead of the aircraft.
The use oftail wheelgearcan pose another difficulty. When
landing,tail wheelaircraft can easily ground loop. A ground
loop is when the tail of the aircraft swings around and comes
forward of the nose ofthe aircraft.The reasonthis happens is
due to the two main wheels being forward of the aircraft’s
centerofgravity.The tailwheel is aft of the centerofgravity.
If the aircraft swerves upon landing,the tail wheel can swing
out to the side ofthe intended path of travel. If far enough to
the side, the tail can pull the center of gravity out from its
desired location slightly aft of but between the main gear.
Once the centerofgravity is no longer trailing the mains, the
tail of the aircraft freely pivots around the main wheels
causing the ground loop.
Conventional gear is useful and is still found on certain models
of aircraft manufactured today, particularly aerobatic aircraft,
crop dusters, and aircraft designed for unpaved runway use. It is
typically lighter than tricycle gear which requires a stout, fully
shock absorbing nose wheel assembly. The tail wheel
configuration excels when operating out of unpaved runways.
With the two strong main gear forward providing stability and
directional control during takeoff roll, the lightweight tail wheel
does little more than keep the aft end of the fuselage from
striking the ground. As mentioned, at a certain speed,
1-37

the air flowing over the elevator is sufficient for it to raise
the tail off the ground. As speed increases further, the two
main wheels under the center of gravity are very stable.
Tricycle Gear Configuration
Tricycle gearis the most prevalent landing gearconfiguration
in aviation.In addition to the main wheels,a shockabsorbing
nose wheel is at the forward end of the fuselage. Thus, the
centerofgravity is then forward of the main wheels. The tail
of the aircraft is suspended off the ground and clear view
straight ahead from the cockpit is given. Ground looping is
nearly eliminated since the center of gravity follows the
directional nose wheel and remains between the mains.
Light aircraft use tricycle gear, as well as heavy aircraft. Twin
nose wheels on the single forward strut and massive multistrut/
multiwheel main gear may be found supporting the world’s
largest aircraft, but the basic configuration is still tricycle. The
nose wheel may be steered with the rudder pedals on small
aircraft. Larger aircraft often have a nose wheel steering wheel
located off to the side of the cockpit. Figure 1-87 shows aircraft
with tricycle gear. Chapter 13, Aircraft Landing Gear Systems,
discusses landing gear in detail.
Maintaining the Aircraft
Maintenance of an aircraft is of the utmost importance for
safe flight.Certificated technicians are committed to perform
timely maintenance functions in accordance with the
manufacturer’s instructionsand under Title 14 of the Code of
FederalRegulations (14 CFR). At no time is an act of aircraft
maintenance taken lightly or improvised. The consequences
of such actioncould be fatal, and the technician could lose his
or her certificate and face criminal charges.
Airframe, engine, and aircraft component manufacturers are
responsible fordocumentingthe maintenance proceduresthat
guide managers and technicianson whenand how to perform
maintenance on their products. A small aircraft may only
require a few manuals, including the aircraft maintenance
manual. This volume usually contains the most frequently
used information required to maintain the aircraft properly.
The Type Certificate Data Sheet (TCDS) for an aircraft
also contains critical information. Complex and large
aircraft require several manuals to convey correct
maintenance procedures adequately. In addition to the
maintenance manual, manufacturers may produce such
volumes as structural repair manuals, overhaul manuals,
wiring diagram manuals, component manuals, and more.
Note that the use of the word “manual” is meant to include
electronic as well as printed information. Also, proper
maintenance extends to the use of designated tools and
fixtures called out in the manufacturer’s maintenance
documents. In the past, not using the proper tooling has
caused damage to critical components,which subsequently
failed and led to aircraft crashes and the loss of human life.
The technician is responsible for sourcing the correct
information, procedures, and tools needed to perform
airworthy maintenance or repairs.
Standard aircraft maintenance procedures do exist and can be
used by the technician when performing maintenance or a repair.
These are found in the Federal Aviation Administration (FAA)
approved advisory circulars (AC) 43.13-2 and AC 43.13-1. If not
addressed by the manufacturer’s literature, the technician may
use the procedures outlined in these manuals to complete the
work in an acceptable manner. These procedures are not specific
to any aircraft or component and typically cover methods used
during maintenance of all aircraft. Note that the manufacturer’s
instructions supersede the general procedures found in AC 43.13-
2 and AC 43.13-1.
All maintenance related actions on an aircraft or component
are required to be documented by the performing technician
in the aircraft or component logbook.Light aircraft may have
only one logbook for all work performed. Some aircraft may
Figure 1-87. Tricycle landing gear is the most predominant landing gear configuration in aviation.
1-38

have a separate engine logbook for any work performed on
the engine(s).Otheraircraft have separate propellerlogbooks.
Large aircraft require volumes ofmaintenance documentation
comprised ofthousands ofproceduresperformed by hundreds
of technicians. Electronic dispatch and recordkeeping of
maintenance performed on large aircraft such as airliners is
common. The importance of correct maintenance
recordkeeping should not be overlooked.
Location Numbering Systems
Even on small, light aircraft, a method of precisely
locating each structural component is required. Various
numbering systems are used to facilitate the location of
specific wing frames, fuselage bulkheads, or any other
structural members on an aircraft. Most manufacturers use
some system of station marking. For example, the nose of
the aircraft- may be designated “zero station,” and all other
stations are located at measured distances in inches behind
the zero station. Thus, when a blueprint reads “fuselage
frame station 137,” that particular frame station can be
located 137 inches behind the nose of the aircraft.
To locate structures to the right or left of the center line of
an aircraft, a similar method is employed. Many
manufacturers consider- the center line of the aircraft to be
a zero station from which measurements can be taken to
the right or left to locate an airframe member. This is often
used on the horizontal stabilizer and wings.
The applicable manufacturer’s numbering system and
abbreviated designations or symbols should al-ways be
reviewed before attempting to locate a structural member.
They are not always the same. The following list includes
location- designations typical of those used by many
manufacturers.
• Fuselage stations (Fus. Sta. or FS) are numbered in
inches from a reference or zero point known as the
reference datum. [Figure 1-88] The referencedatumis
an imaginary vertical-plane at or near the nose of the
aircraft from which all fore and aft distances- are
measured.The distanceto a given point is measuredin
inches parallel-to a center line extending through the
aircraft from the nose through the center of the tail
cone. Some manufacturers may call the fuselage
station a body station,- abbreviated BS.
• Buttock line or butt line (BL) is a vertical reference
plane down the center of the aircraft from which
measurements left orright can be made. [Figure 1-89]
• Water line (WL) is the measurement of height in
inches perpendicular from a horizontal plane
usually located at the ground, cabin floor, or some
other easily referenced location. [Figure 1-90]
• Aileron station (AS) is measured out-board from,
and parallel to, the inboard edge of the aileron,
perpendicular to the rear beam of the wing.
• Flap station (KS) is measured perpendicular to the
rear beam of the wing and parallel to, and outboard
from, the inboard- edge of the flap.
• Nacelle station (NC or Nac. Sta.) is measured either
forward of or behind the front spar of the wing and
perpendicular- to a designated water line.
In addition to the location stations listed above, other
measurements are used, especially on large aircraft. Thus,
there may be horizontal stabilizer stations (HSS), vertical
stabilizer stations (VSS) or powerplant stations (PPS).
[Figure 1-91] In every case, the manufacturer’s
terminology and station lo-cation system should be
consulted before locating a point on a particular aircraft.
Another method is used to facilitate the location of aircraft
components on air transport aircraft. This involves dividing
the aircraft into zones. These large areas or major zones
FS −97.0
FS −85.20
FS −80.00
FS −59.06
FS −48.50
FS −31.00
FS −16.25
FS 0.00
FS 20.20
FS 37.50
FS 58.75
FS 69.203
WL 0.00
FS 273.52
FS 234.00
FS 224.00
FS 214.00
FS 200.70
FS 189.10
FS 177.50
FS 154.75
FS 132.00
FS 109.375
FS 89.25
Figure 1-88. Thevarious fuselage stations relative to a single point of origin illustrated in inches or some other measurement (if of
foreign development).
1-39

BL 21.50
BL 47.50
BL 96.50
BL 76.50
BL 61.50
BL 47.27
BL 34.5
BL 21.50
BL 47.50
BL 96.50
BL
86.56
BL 76.50
BL 61.50
BL 47.27
BL 34.5
BL 23.25
BL 16.00
the wing.The uppersurface ofthe wings typically havefewer
access panels because a smooth surface promotes better
laminar airflow, which causes lift. On large aircraft,
walkways are sometimes designated on the wing upper
surface to permit safe navigationby mechanics andinspectors
to critical structures andcomponents locatedalong thewing’s
leading and trailing edges. Wheel wells and special
componentbays are places where numerous components and
accessories are groupedtogetherforeasy maintenance access.
Panels and doors on aircraft are numbered for positive
identification.On large aircraft, panels are usually numbered
sequentially containingzone and subzone information in the
panelnumber.Designationfor a left or right side location on
the aircraft is often indicated in the panel number. This could
be with an “L” or “R,” or panels on one side of the aircraft
could be odd numbered and the other side even numbered.
Figure 1-89. Butt line diagram of a horizontal stabilizer.
are further divided into sequentially numbered zones and
subzones. The digits of the zone number are reserved and
indexed to indicate the location and type of system of
which the component is a part. Figure 1-92 illustrates
these zones and subzones on a transport category aircraft.
Access and Inspection Panels
Knowing where a particularstructure orcomponent is located
on an aircraft needs to be combined with gaining access to
that area to performthe required inspections or maintenance.
To facilitate this,accessandinspection panels are located on
most surfaces of the aircraft. Small panels that are hinged or
removable allow inspection and servicing. Large panels and
doors allowcomponentsto be removed and installed, as well
as human entry for maintenance purposes.
The underside of a wing, for example, sometimes contains
dozens of small panels through which control cable
components can be monitored and fittings greased. Various
drains and jack points may also be on the underside of
The manufacturer’s maintenance manual explains the
panel numbering system and often has numerous diagrams
and tables showing the location of various components and
under which panel they may be found. Each manufacturer
is entitled to develop its own panel numbering system.
Helicopter Structures
The structures of the helicopter are designed to give the
helicopter its unique flight characteristics. A simplified
explanation of how a helicopter flies is that the rotors are rotating
airfoils that provide lift similar to the way wings provide lift on a
fixed-wing aircraft. Air flows faster over the curved upper
surface of the rotors, causing a negative pressure and thus, lifting
the aircraft. Changing the angle of attack of the rotating blades
increases or decreases lift, respectively raising or lowering the
helicopter. Tilting therotor plane of rotation causes the aircraft to
move horizontally. Figure 1-93 shows the major components of a
typical helicopter.
Airframe
The airframe, or fundamentalstructure,ofa helicoptercan be
made of either metal or wood composite materials, or some
WL 123.483
WL 97.5
WL 79.5 WL 73.5
WL 7.55 Ground line WL 9.55
Figure 1-90. Water line diagram.
1-40

379
411
437
2
1
5
1
1
.
536
5568.585
2
6
4
6
5
2
.
8843.
863
886
903
9
4
3
C FUS-WING STA 0
L
15.2
16.5
25.7
41.3
65.7 NAC C 56.9
L
72.5
76.5
BL 86.179
85.5 88.1
106.4
2°
104.1
111
127.2
652.264
674.737
122
62
5.
30
148 177.0
15° 163
178
F
S
F
S
FS
199
FUS C
220
L
WGLTS 49.89
242
258
NAC CL
264
WGLTS 0.00
274
282 BL 86.179
294.5 2°
353
3
7
1
315.5
329.5
100.72
1
4
1
5
1
.
1771
8
5
200
218.1
7
343.5
353
371
1
3
5
.
8
4
5
315155.
230.
131
4°
Figure 1-91. Wing stations are often referenced off the butt line, which bisects the center of the fuselage longitudinally. Horizontal
stabilizer stations referenced to the butt line and engine nacelle stations are also shown.
ZONE 300—Empennage 326 324 ZONE 300—Empennage
341
34
2
325
322
323
343 344
345
335
334
351
Zone 600—Right wing
Zone 400—Engine nacelles
Zone 200—Upper half of
fuselage
Zone 700—Landing gear and landing gear doors
ZONE 100—Lower half of fuselage
132
123 133
321
143
311
144
142
312
146
145
333
332
331
ZONE 800—Doors 825
824
822
823
821
811
Zone 500—Left wing
122
134
111 112 121
141
131 Zones
135 Subzones
Figure 1-92. Largeaircraft are divided into zones and subzones for identifying the location of various components.
1-41

combination of the two. Typically, a composite component
consists of many layers of fiber-impregnated resins,
bonded to form a smooth panel. Tubular and sheet metal
substructures are usually made of aluminum, though
stainless steel or titanium are sometimes used in areas
subject to higher stress or heat. Airframe design
encompasses engineering, aerodynamics, materials
technology, and manufacturing methods to achieve
favorable balances of performance, reliability, and cost.
Fuselage
As with fixed-wing aircraft, helicopter fuselages and tail
booms are often truss-type or semimonocoque structures
of stress-skin design. Steel and aluminum tubing, formed
aluminum, and aluminum skin are commonly used.
Modern helicopter fuselage design includes an increasing
utilization of advanced composites as well. Firewalls and
engine decks are usually stainless steel. Helicopter
fuselages vary widely from those with a truss frame, two
seats, no doors, and a monocoque shell flight compartment
to those with fully enclosed airplane-style cabins as found
on larger twin-engine helicopters. The multidirectional
nature of helicopter flight makes wide-range visibility
from the cockpit essential. Large, formed polycarbonate,
glass, or plexiglass windscreens are common.
Landing Gear or Skids
As mentioned, a helicopter’s landing gear can be simply a
set of tubular metal skids. Many helicopters do have
landing gear with wheels, some retractable.
Powerplant and Transmission
The two most common types of engine used in helicopters
are the reciprocating engine and the turbine engine.
Reciprocating engines, also called piston engines, are
generally used in smaller helicopters. Most training
helicopters use reciprocating engines because they are
relatively simple and inexpensive to operate. Refer to the
Pilot’s Handbook of Aeronautical Knowledge for a
detailed explanation and illustrations of the piston engine.
Turbine Engines
Turbine engines are more powerful and are used in a wide
variety ofhelicopters.Theyproduce a tremendous amount of
power for their size but are generally more expensive to
operate. The turbine engine used in helicopters operates
differently than those used in airplane applications. In most
applications, the exhaust outlets simply release expended
gases and do not contribute to the forward motion of the
helicopter. Because the airflow is not a straight line pass
through as in jet engines and is not used for propulsion, the
Tail rotor
Tail boom
Main rotor hub assembly
Stabilizer
Main rotor blades Pylon
Tail skid
Pow erplant
Airframe
Fuselage
Transmission
Landing gear or skid
Figure 1-93. Themajor components of a helicopter are the airframe, fuselage, landing gear, powerplant/transmission, main rotor
system, and antitorque system.
1-42

cooling effect of the air is limited. Approximately 75
percent of the incoming airflow is used to cool the engine.
The gas turbine engine mounted on most helicopters is
made up of a compressor, combustion chamber, turbine,
and accessory gearbox assembly. The compressor draws
filtered air into the plenum chamber and compresses it.
Common type filters are centrifugal swirl tubes where
debris is ejected outward and blown overboard prior to
entering the compressor, or engine barrier filters (EBF), a
paper element type filter, encased in a frame with a
screen/grill over the inlet, and usually coated with an oil.
This design significantly reduces the ingestion of foreign
object debris (FOD). The compressed air is directed to the
combustion section through discharge tubes where
atomized fuel is injected into it. The fuel/air mixture is
ignited and allowed to expand. This combustion gas is
then forced through a series of turbine wheels causing
them to turn. These turbine wheels provide power to both
the engine compressor and the accessory gearbox.
Depending on model and manufacturer, the rpm range can
vary from a range low of 20,000 to a range high of 51,600.
Power is provided to the main rotor and tail rotor systems
through the freewheeling unit which is attached to the
accessorygearboxpoweroutput gear shaft. The combustion
gas is finally expelled through an exhaust outlet. The
temperature of gas is measured at different locations and is
referenced differently by each manufacturer. Some common
terms are: inter-turbine temperature (ITT), exhaust gas
temperature (EGT), or turbine outlet temperature (TOT).
TOT is used throughout this discussion for simplicity
purposes. [Figure 1-94]
Transmission
The transmissionsystemtransfers power fromthe engine to
the main rotor,tail rotor,and otheraccessories during normal
flight conditions. The main components of the transmission
system are the main rotor transmission, tail rotor drive
system,clutch,and freewheeling unit.The freewheeling unit,
or autorotative clutch, allows the main rotor transmission to
drive the tail rotordrive shaft during autorotation. Helicopter
transmissions are normally lubricated and cooled with their
own oil supply. A sight gauge is provided to check the oil
level. Some transmissions have chip detectors located in the
sump.These detectors are wired to warning lights located on
the pilot’s instrument panel that illuminate in the event of an
internalproblem.Some chip detectors on modern helicopters
have a “burn off” capability and attempt to correct the
situation without pilot action. If the problem cannot be
corrected on its own, the pilot must refer to the emergency
procedures for that particular helicopter.
Main Rotor System
The rotor system is the rotating part of a helicopter which
generates lift. The rotor consists of a mast, hub, and rotor
blades. The mast is a cylindrical metal shaft that extends
upwards fromand is driven,and sometimes supported,by the
transmission. At the top of the mast is the attachment point
for the rotorblades called the hub. The rotor blades are then
attachedto the hubby any numberofdifferent methods.Main
rotor systems are classified according to how the main
CompressionSection
Gearbox
Turbine Section Combustion Section
Section
Exhaust air outlet
Compressor rotor
Air inlet
Inlet air
Gear
Compressor discharge air Output Shaft
Combustion gases
Exhaust gases
N2 Rotor Stator N1 Rotor
Igniter plug
Fuel nozzle
Combustion liner
Figure 1-94. Many helicopters use a turboshaft engine to drive the main transmission and rotor systems. The main difference between
a turboshaft and a turbojet engine is that most of the energy produced by the expanding gases is used to drive a turbine rather than
producing thrust through the expulsion of exhaust gases.
1-43

Main rotor hub
Main rotor blades
Pitch change links
Blade pitch horns
Main rotor blades
Main rotor mast
Figure 1-95. Four-blade hingeless (rigid) main rotor. The hub is a single piece of forged rigid titanium.
rotor blades are attached and move relative to the main
rotor hub. There are three basic classifications: rigid,
semirigid, or fully articulated.
Rigid Rotor System
The simplest is the rigid rotor system. In this system, the
rotorblades are rigidly attachedto the main rotorhub and are
not free to slide back and forth (drag) or move up and down
(flap). [Figure 1-95] The forces tending to make the rotor
blades do so are absorbed by the flexible properties of the
blade.The pitch of the blades, however, can be adjusted by
rotation about the spanwise axis via the feathering hinges.
Semirigid Rotor System
The semirigid rotor system in Figure 1-96 makes use of a
teetering hinge at the blade attach point. While held in
check from sliding back and forth, the teetering hinge does
allow the blades to flap up and down. With this hinge,
when one blade flaps up, the other flaps down.
Flapping is causedby a phenomenon known as dissymmetry
of lift. As the plane ofrotation ofthe rotorbladesis tilted and
the helicopter begins to move forward, an advancing blade
and a retreating blade become established (on two-bladed
systems).The relative windspeedis greater on an advancing
blade than it is on a retreating blade. This causes greater lift
to be developed on the advancing blade, causing it to rise
up or flap. When blade rotation reaches the point where
the blade becomes the retreating blade, the extra lift is lost
and the blade flaps downward. [Figure 1-97]
Fully Articulated Rotor System
Fully articulated rotor blade systems provide hinges that
allow the rotors to move fore and aft, as well as up and
Coning hinge Teetering hinge
Blade grip Blade grip
Blade pitch
change horn
Pitch link Coning hinge
Sw ash plate
Figure 1-96. Thesemirigid rotor system of the Robinson R22.
1-44

Direction of Flight
Pitch change axis (feathering)
Retreating Side
win
d
Blade tip
speed
Relativ
e
minus
helicopter
speed
(200 knots)
t a t i o n
r o
e
d
B l a
n
t i o
t a
B l a d e r
o
Advancing Side
Blade tip
wind
speed
plus
Relative
helicopter
speed
(400 knots)
Pitch horn
Flap hinge Drag hinge
Damper
ForwardFl ight 100 knots
Figure 1-97. The blade tip speed of this helicopter is
approximately 300 knots. If the helicopter is moving forward at
100 knots, the relative windspeed on the advancing side is 400
knots. On the retreating side, it is only 200 knots. This difference
in speed causes a dissymetry of lift.
down. This lead-lag, drag, or hunting movement as it is
called is in response to the Coriolis effect during rotational
speed changes. When first starting to spin, the blades lag
until centrifugal force is fully developed. Once rotating, a
reduction in speed causes the blades to lead the main rotor
hub until forces come into balance. Constant fluctuations
in rotor blade speeds cause the blades to “hunt.” They are
free to do so in a fully articulating system due to being
mounted on the vertical drag hinge.
One or more horizontal hinges provide for flapping on a
fully articulated rotor system. Also, the feathering hinge
allows blade pitch changes by permitting rotation about
the spanwise axis. Various dampers and stops can be
found on different designs to reduce shock and limit travel
in certain directions. Figure 1-98 shows a fully articulated
main rotor systemwith the features discussed.
Numerous designsand variations on the three types of main
rotorsystems exist.Engineers continually search for ways to
reduce vibration andnoise caused by therotating parts of the
helicopter.Toward that end, the use of elastomeric bearings
in main rotorsystems is increasing. These polymer bearings
have the ability to deformand return to their original shape.
As such, they can absorb vibration that would normally be
transferred by steelbearings.They alsodo not require regular
lubrication, which reduces maintenance.
Some modern helicoptermain rotors have beendesignedwith
flextures. These are hubsandhub components that are made
out of advanced composite materials. They are designed to
take up the forces of blade hunting and dissymmetry of
Figure 1-98. Fully articulated rotor system.
lift by flexing. As such, many hinges and bearings can be
eliminated from the traditional main rotor system. The
result is a simpler rotor mast with lower maintenance due
to fewer moving parts. Often designs using flextures
incorporate elastomeric bearings. [Figure 1-99]
Antitorque System
Ordinarily, helicopters have between two and seven main
rotor blades. These rotors are usually made of a composite
structure. The large rotating mass of the main rotor blades
of a helicopter produce torque. This torque increases with
engine power and tries to spin the fuselage in the opposite
direction. The tail boom and tail rotor, or antitorque rotor,
counteract this torque effect. [Figure 1-100] Controlled
with foot pedals, the countertorque of the tail rotor must be
Figure 1-99. Five-blade articulated main rotor with elastomeric
bearings.
1-45

ati on To
ot
e
r r
q
d u
la
e
B
e
n
u q o
t i
r ta
o
T aero
Bld
Resultant
torque from
Tail rotor thrust
main rotor
blades
Figure 1-100. A tail rotor is designed to produce thrust in a direction
opposite to that of the torque produced by the rotation of the main
rotor blades. It is sometimes called an antitorque rotor.
modulated as engine power levels are changed. This is
done by changing the pitch of the tail rotor blades. This, in
turn, changes the amount of countertorque, and the aircraft
can be rotated about its vertical axis, allowing the pilot to
control the direction the helicopter is facing.
Similar to a vertical stabilizer on the empennage of an
airplane,a fin or pylon is also a common feature on rotorcraft.
Normally, it supportsthe tail rotor assembly, although some
tail rotors are mounted on the tail cone of the boom.
Additionally, a horizontal member called a stabilizer is often
constructed at the tail cone or on the pylon.
A Fenestron® is a unique tail rotor design which is
actually a multiblade ducted fan mounted in the vertical
pylon. It works the same way as an ordinary tail rotor,
providing sideways thrust to counter the torque produced
by the main rotors. [Figure 1-101]
A NOTAR® antitorque systemhas no visible rotor mounted
on the tail boom. Instead, an engine-driven adjustable fan is
located inside the tail boom. NOTAR® is an acronym that
stands for “no tail rotor.” As the speed of the main rotor
changes, the speed of the NOTAR® fan changes. Air is
vented outoftwo long slotson the right sideofthe tail boom,
entraining main rotor wash to hug the right side of the tail
boom, in turn causing laminar flow and a low pressure
(Coanda Effect). This lowpressure causes a force counter to
the torque produced by the main rotor. Additionally, the
remainder ofthe air from the fan is sent throughthe tailboom
to a vent on the aft left side of the boomwhere it is expelled.
This action to the left causes an opposite reaction
Figure 1-101. A Fenestron or “fan-in-tail” antitorque system.
This design provides an improved margin of safety during
ground operations.
to the right, which is the direction needed to counter the
main rotor torque. [Figures 1-102]
Controls
The controls ofa helicopterdifferslightly fromthosefoundin
an aircraft. The collective, operated by the pilot with the left
hand,is pulled up orpusheddownto increase ordecrease the
angle ofattackon all of the rotorblades simultaneously. This
increases ordecreases lift and moves the aircraft up or down.
The engine throttle control is located on the hand grip at the
end ofthe collective.The cyclic is the control “stick” located
between the pilot’s legs. It can be moved in
Downwash
Air jet
Main rotor w ake Lift
Air intake
Rotating nozzle
Figure 1-102. While in a hover, Coanda Effect supplies
approximately two-thirds of the lift necessary to maintain
directional control. The rest is created by directing the thrust
from the controllable rotating nozzle.
1-46

Amt airframe hb_vol_1-32-74 paginas 32 a 74

Amt airframe hb_vol_1-32-74 paginas 32 a 74

Recommended

Recommended

More Related Content

What's hot

What's hot (20)

Similar to Amt airframe hb_vol_1-32-74 paginas 32 a 74

Similar to Amt airframe hb_vol_1-32-74 paginas 32 a 74 (20)

Recently uploaded

Recently uploaded (20)

Amt airframe hb_vol_1-32-74 paginas 32 a 74